1. Field of the Invention
The invention relates to a manufacturing method of a ridge-shaped three-dimensional waveguide and, more particularly, to a method of manufacturing a ridge-shaped 3-dimensional waveguide of a ferroelectric oxide non-linear crystal (hereinafter, also referred to as FONL) which can be used in a wavelength converting device.
2. Description of the Related Art
In a wavelength converting device e.g., a second harmonic generation (SHG) device, an FONL of a high optical constant is used to efficiently generate a second harmonic. The wavelength converting device needs to satisfy a phase matching condition by a fundamental wave and a second harmonic. Various phase matching methods have been tried such as using angle tuning, temperature tuning, and field tuning in a substrate crystal with birefringence used in the wavelength converting device. Also Cerenkov radiation, Quasi phase matching, and the like have been tried.
A bulk crystal of an FONL of a large non-linear optical constant such as LiTaO3 (hereinafter, referred to as LT), LiNbO3 (hereinafter, referred to as LN), KTiOPO4 (hereinafter, referred to as KTP), or the like is used in a substrate of the wavelength converting device. For example, a bulk crystal of a crystal made of Li, Nb, Ta, and O (hereinafter, referred to as LNT) is formed for example by CZ (Czochralski) method in which a crystal rod grows while pulled up from a melting material liquid.
High quality bulk crystal is expensive and is not practical. A method of forming a monocrystal film of LNT on a sapphire substrate by a plasma vapor phase growing method or the like has been developed (JP-B-5-11078). According to this method, Li, Ta, and Nb are oxidized on the sapphire substrate in an oxygen plasma and the LNT monocrystal film is epitaxially grown and deposited.
A ferroelectric material having an increased high optical constant is demanded for the waveguide of the wavelength converting device. The wavelength converting device in which an FONL having compositions expressed by a formula of, for example, K3Li2xe2x88x92xNb5+xxe2x88x92yTayO15+2xe2x88x92x (xe2x88x920.4xe2x89xa6xxe2x89xa60.20, 0xe2x89xa6yxe2x89xa60.33) of a crystal made of K, Li, Nb, Ta, and O (hereinafter, referred to as KLNT) or a crystal made of K, Li, Nb, and O (hereinafter, referred to as KLN) is formed as an epitaxial layer on a substrate by a metal-organic chemical vapor phase deposition or epitaxy (hereinafter, referred to as MOCVD) method or the like has also been developed (JP-A-8-6083).
In an SHG device using an FONL such as KLNT, KLN, KN, LNT, LN, or the like as the wavelength converting device, the following methods (1) to (3) of forming a 3-dimensional waveguide to strongly confine an injected fundamental light have been tried in order to raise the converting efficiency to convert a fundamental light wavelength into a blue light wavelength.
(1) For example, in a diffusion waveguide of Ti, by diffusing Ti into the crystal surface of an LN substrate, a refractive index of the diffusing portion is increased more than that of a substrate cladding, and a 3-dimensional waveguide is formed, or in a proton converting method, by immersing the LN crystal substrate in phosphoric acid, a conversion from Li+ to proton (H+) occurs, and a layer of a higher refractive index than that of the substrate cladding is obtained on the crystal surface, thereby obtaining a 3-dimensional waveguide.
(2) There is also a loading type 3-dimensional waveguide such that when a dielectric material is loaded onto a part of a two-dimensional waveguide which has previously been formed, an equivalent refractive index of the dielectric loading portion is higher than that of the substrate cladding, and the light can be confined in this portion.
(3) Further, a method of manufacturing a channel type waveguide by polishing, cutting or the like has also been tried.
The above methods of forming 3-dimensional waveguides of the oxide non-linear crystal type, however, have the following problems. According to the forming method of the waveguides of the diffusion type and the proton converting type (chemical working type) of (1), although a smooth waveguide having a low loss can be easily formed, there are problems such that the inherent characteristics of the crystal deteriorate, the non-linear constant to decide the efficiency of SHG decreases, light damage occurs, and the like. According to the method of forming the loading type waveguide of (2), although there is an advantage such that the forming method is easy and no damage is given to the crystal itself is not damaged, since the refractive index difference is small, light confinement ability is weak and a beam profile of the propagation light is distorted, so that converting efficiency deteriorates. Further, the polishing, cutting method of (3) has drawbacks such that the production rate is low, it is difficult to precisely form the waveguide in accordance with the initial design, and the like.
A method of easily forming a ridge-shaped 3-dimensional waveguide in which a channel of a projecting portion having a refractive index higher than that of a substrate cladding is formed has been tried, since the ridge-shaped 3-dimensional waveguide strongly confines an injected fundamental light therinto. A reactive ion etching (hereinafter, referred to as RIE) is used as one of the methods of forming the ridge-shaped 3-dimensional waveguide. RIE is a directed chemical process in which chemically active ions are accelerated along electric field lines to meet a substrate to be worked i.e., a physically and chemically etching process utilizing a physical sputter etching effect and a chemical etching effect.
With respect to the physical sputter etching effect, any material can be etched in the RIE principle. There are, however, problems such that the etching speed is low, the material to be worked is damaged, and the like. Since FONL is generally very rigid, it is difficult to select a material for a mask used in the RIE for processing the FONL such that a high selective etching ratio is given.
With respect to the chemical etching effect, an active ion gas to be used must cause a chemical reaction with a material to be worked. Not only does the ion gas react but also a product generated thereby must be removed. That is, it is necessary that the reactive product has a high vapor pressure.
The present invention has been made in view of the above circumstances, and thus an object thereof is to provide a method of manufacturing a ridge-shaped 3-dimensional waveguide of an FONL which is difficult to be mechanically and chemically worked.
According to the invention, there is provided a method of manufacturing a ridge-shaped 3-dimensional waveguide, comprising the steps of:
preparing a substrate made of a first ferroelectric oxide non-linear crystal;
forming a crystal film made of a second ferroelectric oxide non-linear crystal having a refractive index higher than that of the first ferroelectric oxide non-linear crystal on said substrate;
forming a metal film on said crystal film;
forming a mask by etching said metal film; and
forming a ridge portion by selectively removing said crystal film through said mask by a dry etching method.
In accordance with an aspect of the invention, the first ferroelectric oxide non-linear crystal for the substrate is a material selected from a crystal group consisting of a crystal made of K, Li, Nb, Ta, and O, and a crystal made of K, Li, Nb, Ta, and O and doped with at least one element selected from Ta, Rb, Na or at least one of rare earth elements.
In accordance with another aspect of the invention, the second ferroelectric oxide non-linear crystal for the crystal film is another material selected from a crystal group consisting of a crystal made of K, Li, Nb, Ta, and O, and a crystal made of K, Li, Nb, Ta, and O and doped with at least one element selected from Ta, Rb, Na or at least one of rare earth elements.
In accordance with a further aspect of the invention, said step of forming the crystal film provides the crystal film being fabricated through a metal-organic chemical vapor phase epitaxy method.
In accordance with another aspect of the invention, said crystal film is etched by the dry etching method using an etching gas of a mixture comprising CF4 and O2.
In accordance with a further aspect of the invention, said crystal film is etched by the dry etching method using a tray made of a material that is inactive for the etching gas.
In accordance with a still further aspect of the invention, said inactive material is selected from one of quartz, alumina, and sapphire.
In accordance with a still further aspect of the invention, the step of forming the mask includes a step of forming a resist pattern on the metal film by a photolithography method and a step of one of wet etching and dry etching the metal film according to the resist pattern, thereby forming a metal mask.
In accordance with another aspect of the invention, the metal film is aluminum.
In accordance with a further aspect of the invention, the metal film is chromium.